This study continues quantitative and qualitative investigations into the nature of energy metabolism in the fetus. Both an acute and chronic subhuman primate model (the fetal baboon) is being used to extend previous studies of quantitative energy metabolism. Utilizing appropriately placed arterial and venous catheters for sampling of arteriovenous metabolite differences, antipyrine infusion and the radioactive microsphere technique for measuring fetal blood flow and specific organ blood flows, substrate uptake and production by the primate fetus and by specific tissues of the fetus is being evaluated. These studies are being supplemented by similar investigations of specific organ substrate uptake and production in a more chronic fetal lamb preparation. The effect of maternal ketosis upon fetal metabolism is currently under investigation.